Glucoamylase enzymes (glucan 1,4-α-glucohydrolases, EC 3.2.1.3) are starch hydrolyzing exo-acting carbohydrases, which catalyze the removal of successive glucose units from the non-reducing ends of starch or related oligo and polysaccharide molecules. Glucoamylases can hydrolyze both the linear and branched glucosidic linkages of starch (e.g., amylose and amylopectin).
Glucoamylases are produced by numerous strains of bacteria, fungi, yeast and plants. Particularly interesting, and commercially important, glucoamylases are fungal enzymes that are extracellularly produced, for example from strains of Aspergillus (Svensson et al. (1983) Carlsberg Res. Commun. 48:529-544; Boel et al., (1984) EMBO J. 3:1097-1102; Hayashida et al., (1989) Agric. Biol. Chem. 53:923-929; U.S. Pat. Nos. 5,024,941; 4,794,175 and WO 88/09795); Talaromyces (U.S. Pat. Nos. 4,247,637; 6,255,084 and 6,620,924); Rhizopus (Ashikari et al., (1986) Agric. Biol. Chem. 50:957-964; Ashikari et al., (1989) App. Microbiol. Biotech. 32:129-133 and U.S. Pat. No. 4,863,864); Humicola (WO 05/052148 and U.S. Pat. No. 4,618,579) and Mucor (Houghton-Larsen et al., (2003) Appl. Microbiol. Biotechnol. 62:210-217). Many of the genes that code for these enzymes have been cloned and expressed in yeast, fungal and/or bacterial cells.
Commercially, glucoamylases are very important enzymes and have been used in a wide variety of applications that require the hydrolysis of starch (e.g., for producing glucose and other monosaccharides from starch). Glucoamylases are used to produce high fructose corn sweeteners, which comprise over 50% of the sweetener market in the United States. In general, glucoamylases may be, and commonly are, used with alpha amylases in starch hydrolyzing processes to hydrolyze starch to dextrins and then glucose. The glucose may then be converted to fructose by other enzymes (e.g., glucose isomerases); crystallized; or used in fermentations to produce numerous end products (e.g., ethanol, citric acid, lactic acid, succinate, ascorbic acid intermediates, glutamic acid, glycerol and 1,3-propanediol). Ethanol produced by using glucoamylases in the fermentation of starch and/or cellulose containing material may be used as a source of fuel or for alcoholic consumption.
Although glucoamylases have been used successfully in commercial applications for many years, a need still exists for new glucoamylases with altered properties, such as improved specific activity and increased thermostability.
Different mutations have been made in glucoamylases of Aspergillus that enhance thermal stability and specific activity. Reference is made to U.S. Pat. Nos. 6,537,792; 6,352,851; Chen et al. (1996) Prot. Eng. 9:499-505, Chen et al., (1995) Prot Eng. 8:575-582; Fierobe et al. (1996) Biochem. 35:8698-8704; and Li et al., (1997) Prot. Eng. 10:1199-1204. The need still exists for providing glucoamylase variants with altered properties relative to their parent.